Land prep work to enable NaCl in the Linux x64 GN builds.

Land prep work to enable NaCl in the Linux x64 GN builds.

This should get most of NaCl and the PPAPI stuff needed
for NaCl building and linking. There is more work to be
done to get some of the test binaries working (and
probably fill out parts of the NaCl SDK) and possibly
some pnacl support work remaining as well.

NaCl is still disabled by default (set enable_nacl=true
to change). Enabling nacl is still mostly untested and
likely doesn't work at all :).

R=ncbray@chromium.org, brettw@chromium.org
BUG=432959
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/877553008

Cr-Commit-Position: refs/heads/master@{#318180}